1. Field of the Invention
This invention relates to systems for the measurement of metabolic functions and more particularly systems measuring ventilation and pulmonary gas exchange.
2. Description of the Prior Art
It has long been known that analysis of a person's respiratory air provides valuable information relating to the condition of the subject's pulmonary system. The four most commonly measured variables are respiratory volume, oxygen consumption, carbon dioxide production, and respiratory exchange ratio, which is the ratio of carbon dioxide produced to oxygen consumed. Earlier efforts directed towards respiratory gas analysis involve the timed collection of expired breath in rubberized breathing bags, measuring the volume collected, and analyzing the gas composition contained within. Metabolic rates were then calculated from the data obtained. Needless to say, this method was time consuming, subject to great error, and could be performed only by well equipped laboratories. Furthermore, the aforementioned method was not well suited to the measurement of short duration transients in metabolic functions.
Since the data obtained from respiratory gas analysis is so valuable in diagnosing cardiopulmonary dysfunctions and evaluating overall cardiovascular condition, intensive effort has been directed towards automated systems. The intense nationwide interest in running and in physical conditioning in general has sparked further inventive effort in this field. Analysis of ventilation and pulmonary gas exchange provides an objective means for evaluating the effectiveness of various exercise regimes. Improvements and reduction in the size of gas analyzers and digital computers have resulted in the appearance of a number of automated ventilation and pulmonary gas exchange analyzers. These devices range from complicated laboratory systems requiring the use of powerful computers to simpler, less versatile systems for clinical use. Notably lacking in the prior art are systems which meaningfully integrate cardiac and pulmonary data. Since a subject's cardiovascular and pulmonary systems do not function independently, there is a need for testing devices which analyze variables relating to both and which can display the results in a manner which readily yields usable information.
Given the state of the art in gas analyzers and microprocessors, it is a reasonably simple task to construct a metabolic analyzer capable of measuring ventilation volume, carbon dioxide production, oxygen consumption, and respiratory exchange ratio. The simplest system would be merely an updated, automated version of the earliest breathing bag systems previously described. If one is interested only in long-term averaged values for metabolic functions, no particularly difficult technical problems are presented. When one is interested, however, in the transient behavior of ventilatory and metabolic responses, the technical demands are much greater. When one is interested in short interval measurement of metabolic variables, time delays and asynchronicities which are ignored in long-term averaged measurements take on a great significance. The signals from ventilatory flow sensors and gas concentration analyzers represent physiologic variables which are, in reality, synchronized. Delays introduced by sensor response lags, plumbing and the like, can cause great error in the calculation of transient metabolic phenomena. Various hardware schemes for correcting these errors have been proposed so that accurate short-term metabolic analysis can be made.
A common solution is to employ a wideband magnetic recorder to record the various signals as they are produced. The recording can then be played back and the various asynchronous signals aligned for further processing. A similar scheme digitizes signals before storage on a magnetic medium. The complexity and cost of the aforementioned solutions to the time delay problems greatly limit their applicability outside of the research laboratory.
A less complicated solution, disclosed in the prior art, utilizes a combination of analog and digital delay circuits in conjunction with the analog computer to effect time alignment of flow and concentration signals. The time delay introduced in the flow signal in this system is of a fixed duration which must be experimentally derived. Any changes in the physical dimensions of the system or substitution of any components would certainly change the proper time delay value, thus necessitating complete recalibration. The instant invention discloses a simple and elegant means for the time alignment of flow and concentration signals which allows fast and accurate measurement of physiologic transients. In this system there are no fixed time delays nor is there any need for wideband analog magnetic recorders or their digital equivalent.